The present invention relates to ferrules for optical fiber connectors and, more particularly, to CaTiO.sub.3 or CeO.sub.2 --ZrO.sub.2 ferrules having a physical contact (PC) finish.
The use of optical fibers for high speed communication and data transmission via optical signals has become well established. To this end, optical fiber connectors have been developed to allow optical fibers to be coupled to and uncoupled from other optical fibers or optical devices (i.e., sources or receivers of optical signals). Such connectors must provide precise alignment and minimum spacing at connected fiber ends to assure high levels of light transmission. Furthermore, the connectors should be easily and reliably connected, disconnected and reconnected. Examples of such connectors are shown, for example, in U.S. Pat. Nos. 5,101,463; 5,134,677; 5,052,774; 5,212,752; and 5,222,169. As used herein, the term "connector" means an apparatus which allows connection, disconnection and reconnection of two optical fibers or of an optical fiber and an optical device, as opposed to a "splice," which normally connotes a permanent connection between the fibers.
A critical component of optical fiber connectors is the ferrule. Ferrules are used to align, hold, and protect the fragile ends of optical fibers at the point of connection or termination and therefore have precise dimensional tolerances. When the termini of optical fibers are inserted and fixed (usually glued) within ferrules, the probability of damaging them is significantly reduced, especially during the grinding and/or polishing processes required to produce smooth end-faces on fiber termini. When connectors are installed in the field, the lapping and polishing processes are done by hand and are therefore difficult to control. Ferrules are preferably made from materials having a low coefficient of thermal expansion, high elastic modulus, and high hardness. Ceramics are the material of choice because they exhibit the above properties and can be polished to high smoothness and flatness.
A particularly advantageous type of connector is one which utilizes a ferrule having a physical contact (PC) finish. A PC-finished ferrule has a radiused (i.e., convex) contact surface. The contact surface of the ferrule is the surface at which the terminal end of the optical fiber is located and which is brought into contact with the contact surface of another ferrule in order to make an optical connection between the two termini of the connected fibers. The contact surface can also be brought into contact with a device which generates an optical signal, such as a laser or photodiode, thereby allowing the optical fiber which terminates in the ferrule to transmit the signal. Similarly, the contact surface can be abutted against a device which receives an optical signal carried by the ferrule-terminated optical fiber, such as a photocell. Connectors which employ PC-finished ferrules generally provide means (e.g., a spring) for axially biasing the contact surface of the ferrule away from the connector so that, when coupled to, e.g., another connector, the contact surfaces of both ferrules will be urged into contact with one another. When this occurs, the radiused contact surfaces preferably flatten somewhat against one another, thereby improving the liklihood of intimate contact between the fiber termini by spreading the area of Hertzian contact between the contact surfaces of the two ferrules. For example, a pair of typical, PC-finished zirconia ferrules having an outside diameter of 2.5 mm will have a flat contact patch of about 200 .mu.m when pressed against one another by conventional PC connectors.
Historically, alumina was the first ceramic used to make ferrules. With the advent of the PC-finished ferrule, however, the industry recognized the advantages of using zirconia to make ferrules. Zirconia, while somewhat softer than alumina, has a lower elastic modulus and therefore deforms more easily under pressure. The lower modulus thus increases the area of the Hertzian contact between the radiused ferrules to enhance the likelihood that a good optical connection results. When zirconia is used to make molded/sintered articles such as optical fiber ferrules, it must contain small amounts (about 2 to 5 mole %) of a stabilizer to stabilize the tetragonal phase and prevent fracture that would otherwise be caused by shrinkage accompanying the transition to the more thermodynamically stable monoclinic phase during high temperature sintering. Typical stabilizers include yttria (Y.sub.2 O.sub.3), calcium oxide (CaO), and magnesium oxide (MgO). The most common stabilizer is yttria (Y.sub.2 O.sub.3).
Both alumina and yttria-zirconia are much harder than the typical glass fiber termini joined by the connector. This hardness mis-match causes the fiber to polish faster than the ceramic ferrule. Somewhat misleading is the notion that a hard ferrule acts as a "polish stop", i.e., polishing of the fiber end face ceases when it is "polished down" to where it is coplanar with the ferrule surface thus avoiding any fiber "under-cut." It is true that the rate of fiber polishing is greatly reduced when coplanarity is achieved. However, the softer glass fiber often continues to be worn away at a greater rate than the surrounding ceramic ferrule. This produces "undercut" of the fiber end face (i.e., the fiber end is polished below the plane of the ferrule contact surface). Such undercutting is illustrated in FIG. 1 wherein a typical alumina or yttria-zirconia ferrule 1 is shown. The end-face 2 of optical fiber 3 has been polished down below the contact surface 4 of ferrule 1. This results in an air-gap 5 between end-face 2 of optical fiber 3 and contact surface 4 of ferrule 1. Such an air-gap is typically several tenths of micron in depth.
Since the fiber-ferrule endfaces are no longer coplanar, more force must be applied to the ferrules to produce the deformation required to bring the fiber termini into contact. If insufficient force is applied such that the fiber termini are not brought in contact with one another, the resultant air-gap between the fiber termini causes back reflections and other types of signal loss phenomena to occur in the transmitted optical signal. Furthermore, the undercut can trap and retain polishing debris, which later is dislodged and interferes with inter-ferrule/fiber contact resulting in increased spacing between fibers when two ferrules are mated in an optical connector.
One approach to reducing undercut (while beneficially speeding up the polishing process) would be to reduce the hardness mis-match between the optical fiber and ferrule by using a softer ceramic to make the ferrule. However, there has been great reluctance to give up the perceived robustness of zirconia or alumina ceramics, which artisans of ordinary skill believe is required if ferrules are to withstand the rigors encountered in the use environment. Of particular concern in this regard is the repeated coupling/decoupling movements to which a ferrule is subjected when used in an optical fiber connector (as opposed to, e.g., a splicing device in which a ferrule is subjected to a coupling movement only once during the lifetime of the ferrule). In use, connector ferrules are repeatedly inserted and removed from metal or ceramic sleeves known as couplers as they are engaged/disengaged with other ferrules or optical devices. The clearance between the ferrule and coupler is small such that frictional forces are generated between the ferrule and coupler each time the ferrule is inserted and removed from the coupler. Such frictional forces cause wear which, in turn, produces a build-up of debris (wear particles) within the connector. Although a certain amount of wear is inevitable, excessive wear can cause retention of wear particles in the connector assembly which degrades performance by increasing both signal loss and reflectance. It is frequently impractical to clean both sides of a connector every time it is removed from a coupler. It should therefore be possible for a connector that has been properly cleaned to be repeatedly reconnected without being recleaned.
Accordingly, a need exists in the art for improved, softer ceramics which will reduce the hardness mis-match between optical fibers and ferrules without causing premature or high signal loss over time due to excess wear and debris build-up from repeated coupling/decoupling movements at the point of optical connection.